Insights into the well-dispersed nano-Fe₃O₄ catalyst supported by N-doped biochar prepared from steel pickling waste liquor for activating peroxydisulfate to degrade tetracycline

The harmless and high-value added utilization of steel pickling waste liquor (SPWL) is a major challenge in the metallurgical advancement. Herein, well-dispersed nano-Fe3O4 (NF) composite with N-doped biochar as a spatial support (NF/0.5N(3)BC) is synthesized from SPWL and corncob, which has a large specific surface area (159.12 m(2)/g) and mesoporous structure. Besides, it exhibits excellent degradation efficiency (91.56%) and rate constant (0.042 min(-1)) of tetracycline (TC, 50 mg/L) by activating peroxydisulfate (PDS). Both free radicals and non-free radicals contribute to the TC degradation, among which the Fe species, defective edges, C = O, graphitic-N, and pyridinic-N are identified as the possible active sites. Particularly, N3BC cooperates with NF to form an internal electronic field to accelerate the Fe(III)/Fe(II) redox cycle and direct electron transfer process. Further, the possible degradation pathways are proposed based on DFT and HPLC-MS analysis, and the toxicity prediction results verify that NF/0.5N(3)BC + PDS system has an excellent TC degradation/detoxification ability. This study provided a novel strategy for high-value added utilization of SPWL and economically feasible treatment of TC-contaminated wastewater, so as to realizing the goal of "treating waste with waste".